RDMA/usnic: Refactor usnic_uiom_alloc_pd()

Rather than hard-coding pci_bus_type, pass the PF device through to
usnic_uiom_alloc_pd() and retrieve its bus there. This prepares for
iommu_domain_alloc() changing to take a device rather than a bus_type.

Link: https://lore.kernel.org/r/ef607cb3f5a09920b86971b8c8e60af8c647457e.1649169359.git.robin.murphy@arm.com


Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>